1. Field of the Invention
This invention relates to a motion picture decoding system, in which is capable of reproducing and decoding motion picture coded data which is recorded in a record medium such as a compact disc and a hard disc.
2. Description of the Prior Art
Conventionally, since the motion picture data has a remarkably large amount of information, a record medium of which continuous transmission speed is extremely high is required for recording and reproducing the data. For example, video signals of an NTSC television system are currently recorded and reproduced in and from a so-called magnetic tape or an optical disc.
In addition to the foregoing, such means is essential that can encode video signals at a high efficiency for recording and can decode the reproduced signals at a high efficiency, in order to record the video signals in a record medium having smaller sizes and smaller information recording capacity for a long time. In order to comply with such demand, there has been proposed high efficiency coding systems utilizing correlation of the video signals, including an MPEG (Moving Picture Experts Group) system.
In the MPEG system, the difference between the picture frames of the video signals are taken to reduce the redundancy in a time axis direction, and thereafter, orthogonal transforming means such as DCT (discrete cosine transform) is used to reduce the redundancy in a spatial axis direction. In this manner, the video signals are efficiently coded for recording the same in a predetermined record medium.
For reproduction from the record medium in which the video signals coded at a high efficiency are recorded, it is designed to reproduce the video signals by efficiently decoding the signals, e.g., through inverse orthogonal transform.
In general, for the high-speed reproduction from the record medium, in which the efficiently coded video signals are recorded, as described above, decoding is carried out at intervals of few frames, and the decoded signals are supplied at the same speed as the ordinary reproduction for achieving the high-speed reproduction.
However, for the coding by the foregoing MPEG system, the coding is carried out by predicting the motion between the frames, and there exists a frame which cannot be decoded if the decoded image of a frame in the past or future with respect to the current frame does not exist. Therefore, such an disadvantage is caused that the frames may not be freely selected for the high-speed reproduction.
In practice, the frames allowing the direct access and the decoding are intraframe coded frames (will be referred to as "intraframe" hereinafter), each of which is generally contained in ten-odd frames, and thus the reproduction of only the intraframe cannot achieve smooth high-speed reproduction.
In order to overcome the above-noted disadvantages, the high-speed reproduction may be carried out by the decoding of all the frames at the processing speed several times larger than an ordinary speed. However, the processing speed of the decoding cannot be substantially increased several times due to restriction by hardware.